In color science, two colors are called complementary if, when mixed they produce a shade of grey (including white, or even black in a subtractive system). In most perceptual color models, white is towards the center of the color space, and complementary colors will lie roughly opposite each other. The clearest example is the HSV color space, in which complementary colors lie opposite each other on the wheel (or on any circular cross-section).

In most discussions of complementary color, only fully saturated, bright colors are considered. However, under the formal definition, brightness and saturation are also factors. Thus, in the formal CIE 1931 color space, any color of a particular dominant wavelength can be balanced to create white/grey by adding a particular amount (i.e. brightness) of the complementary wavelength, or a different amount of less saturated versions of the complementary wavelength.

The complement of each primary color (red, blue, or yellow) is roughly the color made by mixing the other two in a subtractive system (red + blue = purple; blue + yellow = green; red + yellow = orange). When two compliments are mixed they produce a grey or brown.